1. Field of the Invention
The present invention relates to laser-based light sources, and particularly to laser-based light sources having controlled output light intensity.
2. Description of Prior Art
Semiconductor lasers are used as light sources in many consumer and industrial products such as laser printers, optical communications links, and optical storage systems. Semiconductor lasers are classified mainly as edge-emitting lasers and surface-emitting lasers. Edge-emitting lasers have relatively high threshold current, and surface-emitting lasers are relatively simple and inexpensive to manufacture. Therefore surface-emitting lasers, especially vertical-cavity surface-emitting lasers, are gradually replacing edge-emitting lasers in modern equipment. Surface-emitting lasers must be properly encased to prevent contamination and to minimize fluctuations in operational temperature.
A conventional laser-based light source as disclosed in JP-A-60-088486 is depicted in FIG. 1. A sub-mount 51 is provided with a laser diode 5 electrically and physically mounted thereon. The sub-mount 51 and an optical detector 6 are secured to a header 4, and enclosed by a can 3. A beam splitter 7 is fixed at a window 8 defined in a top of the can 3. Two electrical conductors 1, 2 respectively extend from the header 4, and electrically connect the laser diode 5 to a controlling circuit (not shown). Transmitted beams (not labeled) emitted from the laser 5 are divided by the beam splitter 7 into a series of diffraction beams (not labeled) serving as a signal-light source, and a series of reflected feedback-light beams (not labeled). The reflected feedback-light beams are collected and converted into electrical signals by the optical detector 6. The electrical signals are transferred to the controlling circuit via the electrical conductors 1, 2, to control the output power of the laser 5.
FIG. 2 shows a relationship between driving current I of the light source structure and output power P1 of the laser 5, at different operational temperatures T1, T2. As is shown, the P1-I characteristic curve 10 changes to the P1-I characteristic curve 11 when the operational temperature rises from T1 to T2. That is, at a given driving current, the laser 5 generates different output power depending on fluctuations in the operational temperature. In order to provide stable output power of the laser 5, the controlling circuit (not shown) is employed to adjust the driving current and thereby achieve the desired stable output power of the laser 5.
FIG. 3 shows a relationship between reflected light intensity 12 of the laser 5 and a light receiving position X of the optical detector 6, at different operational temperatures T1, T2. As is shown, the light intensity distribution characteristic curve 12 changes to the characteristic curve 13 when the operational temperature rises from T1 to T2. Reflected light intensity received by the receiving area x1-x2 of the optical detector 6 changes, and a ratio of the light intensity received by the optical detector 6 to output power of the laser 5 accordingly also changes. Therefore the controlling circuit cannot control the output power of the laser 5 precisely, and the laser 5 becomes unstable.
In view of the above, it is an object of the present invention to provide a light source in which the output power of a laser can be steadily controlled by employing a diffraction grating.
It is another object of the present invention to provide a light source which can minimize the effect of varying operational temperatures on output power of the light source.
In order to achieve the objects set above, a laser-based light source in accordance with the present invention comprises a laser, two optical detectors symmetrically arranged on opposites sides of the laser, a diffraction grating mounted in a can, and a controlling circuit. A plurality of parallel grooves is defined in a bottom face of the diffraction grating. Each groove has a depth xe2x80x9cd.xe2x80x9d A groove separation xe2x80x9caxe2x80x9d is defined between any two adjacent grooves. A groove cycle xe2x80x9cbxe2x80x9d is defined as a sum of the distance a and a width of any one groove. A light intensity of light beams reflected from the diffraction grating depends on the values of xe2x80x9cdxe2x80x9d, xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d. By selecting a desired duty cycle f=a/b for the diffraction grating, the reflected light beams are converged into xc2x11 order light beams. Almost all the xc2x11 order light beams are collected by the optical detectors, notwithstanding variations in operational temperature. The controlling circuit receives feedback signals from the optical detectors, and precisely controls output power of the light source.
Other objects, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: